The present invention relates to a vertical illuminator for a microscope, and more particularly, to such an illuminator which permits a switching between a vertical bright field illumination and a vertical dark field illumination, by moving an optical element such as a half mirror or an annular reflecting mirror into or out of an illuminating light path.
A vertical illuminator for a microscope is used to illuminate the surface of a specimen from an objective lens side when such a specimen is opaque (for example, when it is formed of a metallic material). It is known that such a vertical illuminator may be used to provide a bright field illumination in which a specimen is illuminated through an objective lens or a dark field illumination in which the specimen is illuminated through an annular reflecting mirror or a lens which is disposed in surrounding relationship with an objective lens.
A vertical illuminator is known in the prior art which permits a selection between a bright and a dark field illumination. By way of example, in one illuminator of the kind described, a half mirror which is used to provide a bright field illumination and an annular reflecting mirror which is used to provide a dark field illumination are integrally formed with a bright field illumination lens and a dark field illumination lens, respectively, so that the latter may be selectively brought into an illuminating light path to provide a desired illumination.
One difficulty experienced with such a vertical illuminator results from a large difference in the brightness of a field being observed when the illumination is switched from the dark to the bright field illumination. In particular, when observing a specular specimen which exhibits a high optical reflectivity, the resulting difference is so high that the eyes of a viewer may be damaged. In addition, each time the illumination is changed, the brightness of the field being observed may be adjusted, which is inconvenient.
To accommodate for this, there is proposed in Japanese Laid-Open Patent Application No. 103,362/1979 an illuminator which permits a light flux adjusting filter to be detachably disposed in the light path when the bright field illumination is utilized. However, this results in a troublesome operation since the filter must be moved into or out of the light path each time the illumination is changed.
As disclosed in Japanese Laid-Open Patent Application No. 40,149/1977, there is also proposed an illuminator which inserts a filter into the light path whenever the bright field illumination is used. However, this prevents a sufficient illumination from being provided, because of the presence of the filter in the light path, when it is desired to provide an increased magnitude of illumination for taking pictures.
Another difficulty found in conventional vertical illuminators occurs when changing from the dark to the bright field illumination or vice versa. In this instance, any slight displacement of the annular reflecting mirror, which is used to provide the dark field illumination, from the optical axis of an objective lens prevents the illuminating light from the mirror from producing a dark field illumination. Specifically, part of the illuminating light is allowed to impinge upon the surface of a specimen directly without being intercepted by a light shielding sleeve, allowing the light reflected by the surface to travel into the field of view, thus disadvantageously causing temporary glaring of part of the field of view. While such disadvantage represents a temporary phenomenon which occurs during the switching of the light path, the eyes of a viewer may be damaged if the switching takes place during the time the viewer is observing the specimen.